Solid state imagers typically include a photosensor array coupled to a scintillating medium. Radiation absorbed in the scintillator generates optical photons which in turn pass into a photosensor, such as a photodiode, in which the optical photon is absorbed and an electrical signal corresponding to the incident optical photon flux is generated. Substantially hydrogenated amorphous silicon (a-Si) is commonly used in the fabrication of photosensors due to the advantageous photoelectric characteristics of a-Si and the relative ease of fabricating such devices. In particular, photosensitive elements, such as photodiodes, can be formed in connection with necessary control or switching elements, such as thin film transistors (TFT's), in a relatively large array. In such arrays both the TFF's and the photodiodes typically comprise a-Si.
The performance of amorphous silicon based imagers can be degraded by a number of factors, including, for example, exposure to moisture (which can cause the leakage of a-Si photodiodes to increase irreversibly), exposure to materials, such as solvents used in the fabrication process, that degrade the electrical characteristics of the a-Si photodiode or may damage the polymeric dielectric materials, or by exposure to temperatures higher than those of the a-Si deposition processes (e.g., greater than about 250.degree. C.). It is thus beneficial to use fabrication processes that do not present adverse environmental conditions and to provide a protective boundary for a-Si components to minimize degradation both in fabrication and during operation of the imager. A protective boundary disposed between the a-Si components of the photosensor array and the scintillator in an imager desirably provides 1) protection of the photosensor array from contamination by the scintillating medium (and vice versa); 2) a good surface to which the scintillator material can adhere; 3) good optical coupling between the scintillator and the underlying pixels in the imager array (that is, a high degree of transmission of the optical photons with a minimum of scattering of the photons); and, 4) a good environmental barrier to protect the photosensor array, especially from moisture.
In U.S. Pat. No. 4,906,850 to Beerlage, a thick layer of dielectric, such as silicon nitride or silicon oxide, is deposited over the photosensor array such that it can be patterned to form a number of islands on which a scintillator can be vapor deposited so that it grows up from the upper surfaces of these islands. In the Beerlage device, the grooves cut into the silicon oxide or silicon nitride layer to form the islands are about 10-20 .mu.m wide and 10-20 .mu.m deep, which necessarily implies that the overall protective layer is quite thick (i.e., greater than at least 10 .mu.m). Similarly, the growing scintillator columns from such a large area (i.e., the relatively large top surface of the islands formed in the silicon oxide or silicon nitride) necessitates the use of a high-temperature scintillator deposition process (e.g., about 250.degree. C.) that exposes the a-Si array components and the dielectric material to the same high temperatures. Thick silicon oxide or silicon nitride layers present other problems in imager arrays. For example, such thick layers are prone to crack (both vertically and horizontally), causing poor optical transmission characteristics, such as scattering of light caused by crack channels, and the thick layers are prone to delaminate, causing structural degradation of the imager.
It is thus an object of this invention to provide an imager array in which the a-Si components are protected from environmental elements, such as moisture.
It is a further object of this invention to provide an imager array having a high degree of protection from environmental elements and that is mechanically robust, providing good adhesion between the imager array and the scintillator.
It is a still further object of this invention to provide a method of fabricating an imager array to provide a protective layer and that minimizes exposure to the array to high temperatures in the fabrication process.